The present invention relates to the restoration of engines and, more particularly, to the remanufacture of cast iron block engines and resin impregnated aluminum block engines having cast iron cylinder liners using thin walled prefinished and semifinished cylinder liners which are adhesively retained in a non-interference fit relationship within the rebored block cylinder or cylinder liners.
Within a reciprocating engine, the space between each piston and its cylinder wall is sealed by several piston rings. Each ring is resiliently urged outward for sealing contact with the cylinder wall is maintained. During engine operation, sliding frictional contact between piston rings and cylinder walls wears the ring contact surfaces and cylinder walls. As wear progresses, compression is more difficult to maintain and, additionally, there is increased oil consumption and increased oil contamination from combustion products forced past the piston rings.
In general, replacement parts and restoration methods are available for large engines and engines which are required to be highly reliable, such as aircraft engines. Many diesel engines are designed and built with replaceable cylinders, and replacement cylinder and piston sets are manufactured and made available for overhauling diesel engines. The same is often true of aircraft engines, particularly large engines. Smaller aircraft engines are usually restored using remanufactured cylinders wherein the inner cylinder surface is plated to build up wall thickness and then machined to the desired dimensions and tolerances. One aspect of such restoration operations is that each is very expensive and can only be justified economically in commercial operations or in situations in which the cost of replacement of the entire engine is prohibitive.
In order to increase fuel economy by reducing overall vehicle weight, several automobile manufacturers have designed and produced engines wherein the cylinder block and heads, crankcase, and oil pan are formed of aluminum alloys. While such aluminum alloys perform adequately in a structural sense, they are too soft to stand up to high speed frictional loads and would wear quickly in such applications. To avoid this, other metals or materials are employed for components subject to friction, such as bearings. In particular, cast iron cylinder liners are employed for frictional sealing engagement with the piston rings. Such cylinder liners for aluminum block engines are referred to as "wet cylinder" liners because the engine coolant directly contacts the outer surfaces of such liners to carry away heat from engine combustion and piston friction.
Occasionally, during the operation of a piston engine, the piston rod can become separated from either the piston or the crankshaft. This is referred to as "throwing a rod" and can be very damaging to cylinder walls and to other components within the engine. The usual result is a gouging of the cylinder wall. If the gouge is not too deep, the cylinder can often be repaired by boring out the affected cylinder and the insertion of a replacement cylinder which is then bored and honed to the required dimensions within the block.
Because it is necessary for all the cylinders of an engine to have the same volumetric displacement for engine balance purposes and since it is not economical to bore out and reline the remaining cylinders, the usual procedure is to redimension all the cylinders, including the repaired cylinder, to a greater radius than the original dimensions and to install oversized pistons. In older engines which were not computer controlled and wherein emission standards were not so strict, such repair methods were usually adequate. However, such relining of cylinders of modern engines which changes the displacement of the engine causes operational problems since the control computer and pollution controls are set up for controlling the operation of an engine with a given displacement. Modern engine control computers can often detect differences in pollution control device performance resulting from as little as a one or two inch increase in displacement. Such a detected fault can cause a permanently illuminated "check engine" indicator, making such a rebuilt engine difficult to guarantee. Solution of this problem can require the expense of replacing pollution control devices, such as the catalytic converter.
The conventional liner insertion process is to force a liner into the rebored cylinder using a hydraulic press. This results in an "interference" fit wherein the outer surfaces of the liner frictionally engage the inner surfaces of the rebored cylinder. Thus, an interference fit of a liner within a cylinder generates a radially outward pressure of the liner wall against the cylinder surface. In an engine block not originally designed for relining of the cylinders, an interference fit of a liner can stress and even distort the block. The cumulative distortion of relining all the cylinders in a block can result in rendering the block nonrebuildable.
Because conventional automobile engines with cast iron blocks are manufactured in mass quantities, it is currently not economical to restore the worn cylinders of an engine block to their original specifications. In engines for which cylinder replacement is designed from the outset, the cylinder structures are relatively thick walled. This facilitates machining and honing of the inner cylinder surface since the cylinder structures are self-supporting and stiff enough that there is virtually no distortion of the cylinder wall during machining operations. This allows quick and accurate dimensioning and finishing of the inner cylinder surface. Such engines and replacement cylinders are also designed for relatively convenient replacement of worn cylinders without machining of the cylinder block. In the case of diesel engines, an upper lip of the cylinder engages a shoulder groove in the block while the lower end engages a similar shoulder. The cylinder is then held in place by the cylinder head. Adhesives are often used to seal between the upper and lower rims of the cylinder and the block to retain coolant within the water jacket of the cylinder block which otherwise would likely leak out of the water jacket and probably into the oil pan, contaminating the oil.
Conventional automobile engines with cast iron blocks are not designed for cylinder replacement. The cylinder structure is cast as an integral part of the cylinder block and machined to the required cylinder dimensions. When cylinder repair is required, relatively thin walled cylinder liners are used. In such a case, machining of the cylinder liner occurs with the liner located within the block. The liner wall is, thus, supported by the portion of the original cylinder wall which remains. Because of the relative thinness of the walls of such liners and the difficulty of accurately machining and finishing the inner surface thereof, it has heretofore been considered impractical to supply such liners in a semifinished or prefinished condition, which would otherwise economize the restoration of such engine blocks to their original displacement specifications.
There is another type of aluminum block engine with an aluminum head which is formed by a lost foam casting process and which is resin impregnated for liquid retention. The engine uses relatively thick walled cast iron cylinder liners interference fit within the aluminum cylinder bores which are cryogenically cold shrunk prior to insertion. Currently, such engines are provided in Saturn automobiles (General Motors) and may be provided in other cars in the future. It is reported that the cylinder liners of such engines can be overbored a maximum of 0.015 inch diametrically for rebuilding purposes. However, in a conventional engine rebuilding process, an overbore on the order of 0.020 inch is typical. Additionally, such a maximum allowable overbore would only for the engine to be rebuilt one time. Thus, as designed, such an engine cannot be rebuilt using conventional techniques.
A theoretical alternative to conventional rebuilding techniques is to heat the block to 400.degree.-450.degree. F. and drive the old liners out. The differential thermal expansion rates of aluminum and iron loosens the grip of the aluminum cylinder bores on the iron liners. New iron liners, with the original manufacturer's specified diameter, could then be inserted using a cold shrink process. However, heating the block to such a temperature, which is considerably higher than normal operating temperatures, destroys the resin impregnation, which results in a porous block which will not reliably hold oil and coolant.
Reimpregnation of the aluminum block with the resin is not practical because any contaminant of oil, grease, dirt, label adhesives, or paint would cause local imperfections in the impregnation process, such that these areas would remain porous. It would be extremely laborious and, thus, very expensive to even attempt to adequately clean such contaminants from all surfaces and passages of the block and virtually impossible to accomplish. Thus, such a rebuilding process for aluminum block engines with interference fit iron cylinder liners could not be economically carried out or guaranteed.